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Is there any advantage to adding salt to onion while frying itself rather than adding it the last? My friends say it removes moisture from onion.

Is it true? Why is removing moisture from onion important?

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up vote 7 down vote accepted

It is true, in my experience. However, if your recipe does not require salt, you should not be adding it, of course. Removing moisture from onion with the aid of salt helps it reach its desired state (brown or translucent, as desired) faster.

I am most familiar with the use of onion in Indian cooking. I generally add a pinch or two of salt to onion when I start sauteing it. Onion starts sweating as soon as you add salt and start stirring.

The reason that salt draws out moisture from onion is osmotic pressure. The surface of the onion acts as a semipermeable membrane barrier. Since the solution outside the onion is more concentrated with respect to salt, water moves out of the onion to the general liquid side, to balance the osmotic pressure on either side of the onion surface.

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The drawing out of water occurs when you have a salt solution on both sides of a semipermeable membrane. When you put onion, salt and oil in a pan, you don't have a salt solution on the outside of the onion. So no drawing out of water here. I am sure that your onion sweats after you put it on the hot plate and salt it, but it isn't because of the salt. – rumtscho Nov 7 '11 at 13:21
@rumtscho: I figured that the water from the sweating onions would create a thin layer of salty water on the outside. – Jefromi Nov 7 '11 at 15:36

Adding salt while frying onions has two advantages.

  1. It helps to remove the water from onion easily (only if the water is completely removed does the onion get fried).
  2. The salt helps increase the boiling point of water extracted from the onion, which means instead of 100°C water now boils at say 125°C. This high temp helps to cook/fry the onion faster.
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As @Avinash said, salt helps draw water from onion. As the onion cooks in it's own juices we obtain that caramalized or translucent texture. Alternatively, small quantities of water can be added to obtain the same goal, albeit much slower.

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Adding water will tend to steam or even boil the onions for a bit, which might not be what you want. – Jefromi Nov 7 '11 at 15:34

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